Tunable inductance



R. J. CARY, JR 2,448,296

TUNABLE INDUCTANCE Filed Jan. 29, 1946 INVENTOR RAYMOND J. CARYJR. BY

ATToRNEv Patented Aug. 31 1948 TUNABLE INDUCTANCE Raymond J. Cary, Jr., Long Beach, Calif., assignor to the United States of America as represented by the Secretary of War Application January 29, 1946. Serial No. 644,173

1 Claim. 1

Thisinvention relates to electrical apparatus and more particularly to improvements in tunable inductances.

The conventional tuned-circuit inductance for high frequency amplifier circuits is a helical coil. The inductance of this coil may be either fixed or variable. The variable inductance coil usually has a hollow core in which a plug of magnetic material (e. g., polyiron) or non-magnetic material (e. g., brass) may be moved to vary the inductance of the coil.

At very high radio frequencies, the distributed capacity and the capacity to ground of this conventional coil, which together with other stray tube and wiring capacities determine the stray shunt capacitance, may be of sufiicient magnitude to seriously affect the gain-bandwidth factor of the circuit. To obtain the maximum gainbandwidth factor, the distributed capacity of the coil and its capacity to ground must be made as small as possible.

An object of the present invention is to :provide a tunable inductance for use with high frequency amplifier circuits in which the shunt capacity to ground is minimized.

A further object of the invention is to provide such a tunable inductance with a minimum distributed capacity.

A still further object is to provide such a tunable inductance with a readily-adjustable stable tuning control.

Other objects and advantages of the invention will be apparent during the course of the following description.

Essentially the present invention is a mechanism for axially expanding or contracting a helical coil thereby varying its inductance. The coil is wound about a dielectric rod and one end of the coil is rigidly aflixe-d to the dielectric rod. The dielectric rod moves axially within a tubular mount which is rigidly affixed to the amplifier chassis. The other end of the coil is maintained fixed with respect to the chassis. Hence, axial motion of the dielectric rod within the tubular mount will expand or contract the coil axially, and consequently will vary its inductance.

In the accompanying drawing forming a part of this specification, the single figure is a crosssectional view illustrating one embodiment of the invention.

In the figure, numeral I designates a dielectric rod having an external shoulder near one end. While the material of rod D is a dielectric in the present embodiment, other materials may be used, under different requirements. The rod I0 is disposed within a metallic tubular mount Ii. One end of the tubular mount II has an external thread I2 whereby the mount may be secured to a chassis I3. The other end of tubular mount I I has an internal thread I4. The tubular mount II has an internal shoulder beyond the internal thread I4.

A section of rod I0 extends externally beyond tubular mount II. A helical coil I5 is disposed about this external section of rod I0. One end of coil I 5 is rigidly aflixed to a :pin I6 which extends through the end of rod Ill. The other end of coil I5 is fixed to a tie-point pin I1. The pin I1 is swedged to a protrusion on an insulating block I8. Insulating block I8 is maintained in a fixed position with respect to the chassis I3 by means of a screw I9. Hence, it is obvious that the end of coil I5 which is attached to pin I! will be maintained in a fixed position with respect to chassis I3 and tubular mount II. The tie-point pin I1 is here shown as insulated from chassis I3 by means of the insulating block I 8. Pin I! could of course be grounded directly to chassis I3 if desired.

Axial motion of rod ID with respect "to tubular mount II is efiected by an adjusting screw 20 moving within thread I4 and applying pressure to one end of rod ID. The pressure between rod III and screw 20 is maintained by a spring 2| which is disposed about rod I0 and between the external shoulder of rod I0 and the internal shoulder of tubular mount I I.

The inductance of coil I5 may be varied by adjusting the screw 20. One end of coil I5 is maintained fixed by pin I! and therefore axial motion of rod I0 caused by rotation of the adjusting screw 20 will either expand or contract coil I5, thereby changing the axial spacing or pitch of coil I5 and consequently its inductance.

The rod I0 accurately follows the motion of the screw 20 because of the pressure of the spring 2|. The pressure of spring 2| also serves to prevent vibration along the axis of the coil, which vibration if present would vary the inductance of the coil introducing a vibration modulation in the output circuit.

The invention results in having the coil I5 characterized by a low distributed capacity and by a low shunt capacity to ground. The coil I5 may be sufiiciently removed from the chassis I3 to minimize shunt capacity to ground. The possibility of widely spacing the turns on coil I5 in conjunction with the use of the dielectric rod I0 tends to minimize the distributed capacity of coil [5.

It will be apparent that there may be deviations from the invention as described which still fall fairly within the spirit and scope of the invention What is claimed is:

A tunable inductance including a tubular mount having an internal thread at one end thereof, a dielectric rod disposed partly within.

said tubular body portion and extending externally beyond said tubular mount at the other end thereof, screw means associated with said internal thread and adapted to apply pressure to;

the end of said rod which is disposed within said tubular mount, spring meansdisposed" about said rod and adapted to maintain the pressure between said rod and said screw means, a helical coil of conducting material disposed'about the.

external portion of said dielectric rod, one end of said coil being rigidly affixed to said dielectric rod and the other end fixed with respect to said tubular mount, whereby the motionof said screw 4 means within said internal thread will move said dielectric rod axially with respect to said mount thereby varying the pitch of said helical coil and changing the inductance thereof.

RAYMOND J. CARY, JR.

REFERENCES CITED The following references are of record in the 0 file of this patent:

UNITED STATES PATENTS Great Britain June 4,. 1925 

